Hello, My Child Is Underweight And Has An Immune Problem.

Hello, my child is underweight and has an immune problem. He's been to an allergist. He's is currently on Mct oil. He's also on Neocate for formula it's a hypoallergenic milk. He has a bad cough at night. He refuse to eat. He rarely drinks milk the past 2 days. His teeth are coming out also. He has bad eczema occasionally. I don't know what to do any help?
Thank you